The genetics of bay, black and chestnut


Great topic!

I have always been fascinated by colour genetics. There has always been one aspect however that my mind has stumbled over, perhaps someone can clarify for me...

It is the definition of a bay vs. the definition of a brown and where you draw the line between the two? I know some geneticists consider brown to be a type of bay. I tend to feel that the seal browns are separate from bay, but that some "brown" horses with black points are just shades of bay. What do you think?

Also does the agouti series influence chestnut colour? I know that chestnut horses carry agouti and in one of my books it says that the shades of chestnut are caused by agouti, ie. liver, standard orange/brown, red, and sandy chestnut. Has anyone else heard/read this?

 
Originally posted by virtuallyhorses
I would correct the person who said that dorsal stripes are not a dun thing - indeed they are! when you see them in other colours they are the evidence (usually) of a recessive dun gene for instance EeAaCCDnndDnnd would be a bay horse and may or may not show any traits of the recessive Dun gene. The exception is a foal which starts out with a dorsal stripe which then fades.
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But then we are faced with all the duns who are genetically dun, but show no dorsal stripe, along with the 'layman's' dorsal stripe on non-duns, which is in fact a form of counter-shading caused by the sooty gene (probably among others).

A clear, sharp, true dorsal stripe is certainly a dun characteristic - but there are also 'pseudo' dorsal stripes that really just confuse the whole issue.

One of the things that really fascinates me is the dun gene. In spite of the fact that there is, almost without doubt, a simple dominant dun gene, it's also been shown that horses left to breed for several generations without human intervention 'develop' dun colouring. That would support the theory of a reccessive form of dun. Dun as described here is described in the simple dominant form though - mainly to try and keep it simple. In order to register a horse as dun it must show the characteristics of the dominant form of dun - including the need for at least one dun parent.

ShinyMcShine - the answer to that one really depends on which theory you subscribe to. It has been shown in research in France that seal brown actually carry Agouti, but are not considered bay because two Seal browns will never produce a bay foal. Current thinking is that there are in fact four alleles at the Agouti locus instead of simply A or a for bay or not bay.

Thsi would mean that at the Agouti locus we now have - A (bay), a (black), A+ (Wild-type bay - where the black on the legs is mixed with red, so points appear paler - may also be linked with the lightening of flanks and muzzle which may also be caused by or in part by Pangare or Mealy gene) and a^t - Seal brown, where Agouti partially restricts black.

Agouti, because it affects black pigment, doesn't influence chestnut. Chestnut can, however, be influenced by other genes - Pangare, Mealy and Sooty among them. It's possible that liver chestnuts are red base with sooty modifier - again, it doesn't satisfactorily explain all the variations. I have 3 liver mares. One is a dark reddish liver, with mane and tail the same liver colour. One has darkened over the years and now has mane and legs so dark they can easily be mistaken for black at times. The third is a gingery liver with a grey mane and tail. It would seem there is more than one gene at work to cause the variations in shade.
 
Browns are actually quite easy to identify - by their brown noses. Take a look at the horse on the new rider logo and he has that distinctive brown nose of a brown. Many ask whether Imp is a black horse (and although I could answer yes because there is no black\brown genetic distinction) he is quite easily identifiable as brown because of the brown nose (and the red hairs in mane and tail) and of course he will fade to brown in the summer.

A 'black' horse showing the typical brown nose and red hairs of the mane that distinguishes him as a brown.

BTW One of the big things to remember with all this genetic 'science' is that many of the genes and alleles we are discussing are theoretical only! Only a few genes have been identified and the majority of modifiers have not.

Also genes are only part of the story - humans only have two genetic eye colours brown or blue but look at the diversity of eye colour! :)
 

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Originally posted by virtuallyhorses
BTW One of the big things to remember with all this genetic 'science' is that many of the genes and alleles we are discussing are theoretical only! Only a few genes have been identified and the majority of modifiers have not.

Also genes are only part of the story - humans only have two genetic eye colours brown or blue but look at the diversity of eye colour! :)
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Very true. Genetic tests have only actually identified genes for black/red (Extension), Agouti the bay gene (A), cream (called the MATP gene) and frame overo (called the EDNRB gene). Although horses can be tested for tobiano homozygosity, the actual tobiano gene has not been identified yet - the test looks for two protein markers closely linked to the gene, in conjunction with 3 generation pedigree analysis.

All other genes are still, as Viv says, in the realms of scientific theory....

A black horse with a brown nose (like Imp) can be tested to ascertain whether or not it is black or bay, by identifying the red factor. Some horses that are genetically black can appear more brown - sunlight can fade black coats, or they may be carrying a single cream gene that causes the coat to appear a slightly browner black. Generally though if a horse has brown on its muzzle, or near the eyes, or the flank - it's not black.

It's the theory aspect that causes most confusion. Because colour genetics is by and large mostly theory, there are also more than one system of notation in use. Sponenberg, for example, theorises that flaxen is a reccessive gene (similar to chestnut) which causes red mane and tail hair to dilute to blond. He believes this accounts for the colour in the Hafflinger, where flaxen chestnut is the only colour - if both flaxen and chestnut are reccessive that would explain nicely why no other colours occur. Each horse would pass on one copy of e and one copy of f so every foal is homozygous for e (making it chestnut) and f (what Sponenberg terms ruano - flaxen) making it flaxen chestnut. It also explains why two non-flaxen horses can produce a flaxen foal - same way two blacks can produce a chestnut. But this is just theory, and notation used only by Sponenberg, not Bowling, Davis or other geneticists. Another of Sponenberg's theoretical genes is F - the Tostado gene, which causes red points on chestnut horses. Again, based on observation, the theory would seem to be sound - but it is just theory, and until these genes are identified (or the action they supposedly cause attributed to another gene or group of genes) it will remain theory.

Still very interesting though....
 
I read a lot! :D The really sad part is that I know most of this stuff off by heart now.... there are some good books on genetics (not just colour although there's a couple that deal with colour and nothing else too) if you want to learn more.
 
Hoping someone's still watching this thread!

Just a curious question (or two) is brown a dark red or light black - with reference to the base colour being either red or black.

And, why does a bay's coat get lighter and more red in the summer?
 
A brown is a black based horse as is a bay.

Most colours change appearance from season to season. There is no specific genetic "reason" as such known. But there are a lot of factors that will influence the appearance of a colour. Seasonal changes in hair length and diet can be contributing factors.
 
Some red-based horses can appear very dark brown and even be mistaken for black-based horses, just as some black based horses wilol fade out so much they appear red. The difference is that seal brown horses will have black points; red based horses cannot produce black pigment and so will have brown points too.

There are brown horses that don't appear to have black points but who test positive for a black base. Causes vary; some are a particular shade of agouti that seems to restrict black pigment in a different way, some may have another modifier at work (like cream; cream on black usually doesn't give much visual clue to it's presence, but it can also result in a faded brown-looking horse).

As Jaimee says, seasonal variations occur in most colours; sweat and teh sun fades some colours, the diet usually changes to an extent, which will affect coat colour, and winter and summer coats do usually vary in shade anyway.
 
A very fascinating subject. As I am now getting ready to buy some broodmares to start the friesian warmblood breeding programme the subject is even more fascinating.

I ideally would like to breed the horses darker preferably black or as close to black as possible.

Frits is black, his nose is black the only 'red' (more of a maroon colour than red)is at the tip of his tail after summer. In the friesian breed it can cost a stallion a studbook breeding licence if he has the chestnut gene (or active chestnut gene, as I am not sure how it all works). The stallions when given the nod will be tested for this gene.

Now with that thought in mind whilst shopping for broodmares I have been looking for horses that would compliment Frits. I am looking at horses that are bay or brown in the hope that the darker colour of the mares and the black of Frits will produce dark horses. Am I on the right track?

Recently I saw a broodmare that fitted almost all of the criteria except that she was grey. I was not sure how grey to black would work - but I guess I should ask this after I read the grey sticky:)
 
Really confused!:confused:

My grey mare had a chestnut foal. We put the mare to a brown stallion. Dams dam chestnut dam sire grey.
Sire dam dont know sire sire brown

Im looking to put her to a chestnut stallion next time Who sire was bay. What colour am i likely to get??
 
Grey mare only has one copy of grey, which she did not pass on to the foal. She did however have one copy of chestnut, which she passed on to the foal. Stallion also had one copy of chestnut which he pased on; so you get a chestnut foal.

We don't know what colour your mare is under the grey, but we do know she has at least one copy of chestnut, so put to a chestnut stallion you'd have a 50% chance of chestnut at least. There's also a chance she carries black or bay, so you might also get a bay or black foal.
 
Wow this is so interesting:)

I'm dieing to know; I have an appaloosa mare, bred from an appaloosa dam by a bay TB stallion. If I bred her to an appaloosa stallion what are my chances of getting a spotty foal?
 
My horse is very interesting.

Dam piebald, full feathers
Sire piebald, fulls feathers

Foal - Black! No feathers!


After a lot of research, its turns out my mare is a throw-back to her great grand sire, Ard Allez Cat, a pure black TB
 
Welsh_Girl said:
Wow this is so interesting:)

I'm dieing to know; I have an appaloosa mare, bred from an appaloosa dam by a bay TB stallion. If I bred her to an appaloosa stallion what are my chances of getting a spotty foal?
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It depends really. Does your mare have spots? If so, bred to a stallion with spots there's a chance of spots, no spots, or a fewspot (fewspots are homozygous for the spotting gene Lp). There's also another aspect, which is the expression of white - frosting, blankets and so on (PATN1).

Bred to a fewspot stallion guarantees either a spotty foal or a fewspot.

That's a very basic start anyway...

Maverick - your mare is not a throwback. Both her parents have one copy of tobiano, the splodgy gene. In your mare's case, neither passed it on, so she is solid. The first foal I bred from my black and white stallion, to a bay and white mare, was... solid chestnut! There's a 25% chance of a solid foal from two parents who both carry one copy of tobiano.
 
Hi Chev

She's 12 now and still has spots, she's currently grey (white) with black leopard spots. Her dam was marked like this when I saw her at about 20 yoa.

When my horse was born she was black with a white rump and big black "splotches". Another pic i saw of her at five she was blue roan with black flecks through the coat and dark legs. When I bought her 4 years ago she was a very well covered leopard spot (white with lots of black spots of different sizes.

I'd be interested to know why all the changes of colour throughout her life, could it indicate many different appaloosa genes?
 
is it Like eye colours? that u study in Biology and the little chart for blue and brown Eyes
B - Brown
b - blue

BB Bb
Bb bb

So does black and red look like this???

BB Br
Br rr
 
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Sort of. The gene controlling coat colour is the Extension gene. It contains the alleles which say whether a horse can produce black pigment or not. If it can, it'll have at least one E allele. E is a simple dominant, so a horse only needs one copy of E to be black based. If both alleles within the Extension gene are e, it means the horse cannot produce black pigment, only red. Red is reccessive and is noted as ee.

A horse with two alleles that allow black to be produced (EE) will only ever pass E onto a foal, so all foals will be black based. An Ee horse can also pass on red (or, not-black, really) so can produce chestnut based foals. So possibilities are EE, Ee, and ee.

All coat colours other than black and red are down to genes that modify the black or red base. Bay is one; in black horses it restricts black pigment to the points. In chestnut horses it does nothing visually, but means a chestnut can produce a bay foal if put to a black. In black horses, it keeps black to the mane, tail and legs, and leaves the body red. Cream is another that alters the appearance and production of pigment in the coat, as do dun, champagne and silver. They are modifiers. A bay horse might be EE AA, Ee AA, EEAa, or EeAa. A buckskin might be Ee Aa Crcr, or EE Aa Crcr, or EE AA Crcr... you get the idea!

Then there are pattern genes; they control the white and spotting patterns. The KIT gene contains several of these alleles; tobiano, sabino, roan etc all at different loci.

So it's slightly more complicated than blue or brown eyes, but the theory is the same.
 
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